Method of cutting object to be processed

ABSTRACT

A method of cutting an object which can accurately cut the object is provided. An object to be processed  1  such as a silicon wafer is irradiated with laser light L while a light-converging point P is positioned therewithin, so as to form a modified region  7  due to multiphoton absorption within the object  1 , and cause the modified region  7  to form a starting point region for cutting  8  shifted from the center line CL of the thickness of the object  1  toward the front face  3  of the object  1  along a line along which the object should be cut. Subsequently, the object  1  is pressed from the rear face  21  side thereof. This can generate a fracture from the starting point region for cutting  8  acting as a start point, thereby accurately cutting the object  1  along the line along which the object should be cut.

TECHNICAL FIELD

The present invention relates to a method of cutting an object to beprocessed for cutting an object to be processed such as a semiconductormaterial substrate, a piezoelectric substrate, and a glass substrate.

BACKGROUND ART

One of laser applications is cutting. The following is typical cuttingby laser. For example, a part to cut in an object to be processed suchas a semiconductor wafer or glass substrate is irradiated with laserlight having a wavelength absorbed by the object, and melting by heatingis advanced by the laser light absorption from the front face to rearface of the object in the part to cut, so as to cut the object. However,this method also melts the surroundings of a region which becomes a partto cut in the front face of the object. As a consequence, in the casewhere the object to be processed is a semiconductor wafer, there is afear of semiconductor devices positioned near the above-mentioned regionamong those formed on the front face of the semiconductor wafer melting.

Examples of methods of preventing the front face of such an object to beprocessed from melting include laser cutting methods disclosed inJapanese Patent Application Laid-Open Nos. 2000-219528 and 2000-15467.The cutting methods of these publications cause laser light to heat apart to cut in the object to be processed, and then cool the object, soas to generate a thermal shock at the part to cut in the object, therebycutting the object.

DISCLOSURE OF THE INVENTION

When the thermal shock generated in the object to be processed is largein the cutting methods in these publications, however, unnecessaryfractures such as those deviating from the line along which the objectshould be cut or those extending to a part not irradiated with laser mayoccur in the front face of the object. Therefore, these cutting methodscannot perform precision cutting. When the object to be processed is asemiconductor wafer, a glass substrate formed with a liquid crystaldisplay unit or a glass substrate formed with an electrode pattern inparticular, semiconductor chips, the liquid crystal display unit, andelectrode pattern may be damaged by the unnecessary fractures. Since anaverage input energy is large in these cutting methods, they impart alarge thermal damage to the semiconductor chips and the like.

In view of such circumstances, it is an object of the present inventionto provide a method of cutting an object to be processed which canaccurately cut the object.

For achieving the above-mentioned object, the method of cutting anobject to be processed in accordance with the present inventioncomprises a starting point region for cutting forming step ofirradiating a wafer-like object to be processed with laser light whilepositioning a light-converging point therewithin, so as to form amodified region due to multiphoton absorption within the object, andcausing the modified region to form a starting point region for cutting,deviated from a center position of the object in a thickness directionthereof toward one end face of the object, along a line along which theobject should be cut in the object; and a pressing step of pressing theobject from the other end face side of the object.

In this method of cutting an object to be processed, the modified regionformed by multiphoton absorption forms a starting point region forcutting within the object along a desirable line along which the objectshould be cut for cutting the object. Here, the multiphoton absorptionoccurs locally within the object, so that laser light is hardly absorbedby one end face of the object and the other end face on the oppositeside thereof, whereby one end face and the other end face can beprevented from melting upon laser light irradiation. Since the startingpoint region for cutting is formed so as to deviate from the centerposition of the object in the thickness direction thereof toward one endface, when the object is pressed from the other end face side, afracture can be generated in the object from the starting point regionfor cutting acting as a start point with a pressing force smaller thanthat in the case where the starting point region for cutting is formedat the center position. This can prevent unnecessary fractures deviatedfrom the line along which the object should be cut from occurring, andaccurately cut the object along the line along which the object shouldbe cut.

Here, the light-converging point refers to a location at which laserlight is converged. The starting point region for cutting refers to aregion to become a start point for cutting when the object to beprocessed is cut. Therefore, the starting point region for cutting is apart to cut where cutting is to be performed in the object. The startingpoint region for cutting may be produced by continuously forming amodified region or intermittently forming a modified region. Theexpression “form a starting point region for cutting deviated from acenter position of the object in a thickness direction thereof towardone end face of the object” means that a modified region constitutingthe starting point region for cutting is formed so as to deviate fromthe half thickness position of the object in the thickness directionthereof toward one end face. Namely, it means that the center positionof the width of the modified region (starting point region for cutting)in the thickness direction of the object is positioned so as to deviatefrom the center position of the object in the thickness direction towardone end face, and is not limited to the case where the whole modifiedregion (starting point region for cutting) is positioned on the one endface side of the center position of the object in the thicknessdirection.

Preferably, the pressing step presses the object along the line alongwhich the object should be cut. When cutting an object to be processedinto functional devices in the case where the functional devices areformed like a matrix as a laminate part on the other end face of theobject, for example, the object can accurately be cut into thefunctional devices if a line along which the object should be cut is setbetween neighboring functional devices and the object is pressed alongthis line along which the object should be cut. Also, this cansubstantially eliminate the action of the pressing force on thefunctional devices.

Preferably, positional data of the line along which the object should becut with respect to the object to cut is stored in the starting pointregion for cutting forming step, and the object is pressed along theline along which the object should be cut according to the positionaldata in the pressing step. This makes it possible for the pressing forceto act easily and accurately on the starting point region for cuttingformed within the object.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an object to be processed during laserprocessing in the laser processing method in accordance with anembodiment of the present invention;

FIG. 2 is a sectional view of the object to be processed taken along theline II-II of FIG. 1;

FIG. 3 is a plan view of the object to be processed after laserprocessing by the laser processing method in accordance with theembodiment;

FIG. 4 is a sectional view of the object to be processed taken along theline IV-IV of FIG. 3;

FIG. 5 is a sectional view of the object to be processed taken along theline V-V of FIG. 3;

FIG. 6 is a plan view of the object to be processed cut by the laserprocessing method in accordance with the embodiment;

FIG. 7 is a graph showing relationships between the electric fieldintensity and crack spot size in the laser processing method inaccordance with the embodiment;

FIG. 8 is a sectional view of the object to be processed in a first stepof the laser processing method in accordance with the embodiment;

FIG. 9 is a sectional view of the object to be processed in a secondstep of the laser processing method in accordance with the embodiment;

FIG. 10 is a sectional view of the object to be processed in a thirdstep of the laser processing method in accordance with the embodiment;

FIG. 11 is a sectional view of the object to be processed in a fourthstep of the laser processing method in accordance with the embodiment;

FIG. 12 is a view showing a photograph of a cut section in a part of asilicon wafer cut by the laser processing method in accordance with theembodiment;

FIG. 13 is a graph showing relationships between the laser lightwavelength and the internal transmittance of a silicon substrate in thelaser processing method in accordance with the embodiment;

FIG. 14 is a schematic diagram of the laser processing apparatus inaccordance with the embodiment;

FIG. 15 is a flowchart for explaining the laser processing method inaccordance with the embodiment;

FIG. 16 is a plan view of the object to be processed in accordance withExample 1;

FIG. 17 is a sectional view showing a step of making the object to beprocessed in accordance with Example 1;

FIG. 18 is a sectional view showing the starting point region forcutting forming step in accordance with Example 1;

FIG. 19 is a sectional view showing a case where a starting point regionfor cutting is positioned across a center line in the object to beprocessed in accordance with Example 1;

FIG. 20 is a sectional view showing a case where the whole cuttingregion is positioned on the front face side of the center line in theobject to be processed in accordance with Example 1;

FIG. 21 is a sectional view showing a case where a starting point regionfor cutting on the rear face side is positioned on the center linewhereas a starting point region for cutting on the front face side ispositioned between the starting point region for cutting on the rearface side and the front face in the object to be processed in accordancewith Example 1;

FIG. 22 is a sectional view showing the pressing step in accordance withExample 1;

FIG. 23 is a sectional view showing a step of expanding an expansionsheet in accordance with Example 1;

FIG. 24 is a sectional view showing a case where the object to beprocessed is irradiated with laser light from the rear face side thereofin the starting point region for cutting forming step in accordance withExample 1;

FIG. 25 is a sectional view showing the starting point region forcutting forming step in accordance with Example 2;

FIG. 26 is a sectional view showing the pressing step in accordance withExample 2; and

FIG. 27 is a sectional view showing a case where the object to beprocessed is irradiated with laser light from the rear face side thereofin the starting point region for cutting forming step in accordance withExample 2.

BEST MODES FOR CARRYING OUT THE INVENTION

In the following, a preferred embodiment of the present invention willbe explained with reference to drawings. In the starting point regionfor cutting forming step of the method of cutting an object to beprocessed in accordance with this embodiment, the object is irradiatedwith laser light while a light-converging point is positionedtherewithin, so as to form a modified region due to multiphotonabsorption within the object. Therefore, the laser processing method,multiphoton absorption in particular, will be explained at first.

A material becomes optically transparent if its absorption bandgap E_(G)is greater than a photon energy hv. Hence, the condition under whichabsorption occurs in the material is hv>E_(G). However, even whenoptically transparent, the material yields absorption under thecondition of nhv>E_(G) (n=2, 3, 4, . . . ) if the intensity of laserlight is very high. This phenomenon is known as multiphoton absorption.In the case of pulse waves, the intensity of laser light is determinedby the peak power density (W/cm²) of laser light at a light-convergingpoint thereof. The multiphoton absorption occurs, for example, at a peakpower density (W/cm²) of 1×10⁸ (W/cm²) or higher. The peak power densityis determined by (energy per pulse of laser light at thelight-converging point)/(laser light beam spot cross-sectionalarea×pulse width). In the case of a continuous wave, the intensity oflaser light is determined by the electric field strength (W/cm²) oflaser light at the light-converging point.

The principle of laser processing in accordance with the embodimentutilizing such multiphoton absorption will now be explained withreference to FIGS. 1 to 6. FIG. 1 is a plan view of an object to beprocessed 1 during laser processing; FIG. 2 is a sectional view of theobject 1 taken along the line II-II of FIG. 1;

FIG. 3 is a plan view of the object 1 after laser processing; FIG. 4 isa sectional view of the object 1 taken along the line IV-IV of FIG. 3;FIG. 5 is a sectional view of the object 1 taken along the line V-V ofFIG. 3; and FIG. 6 is a plan view of the cut object 1.

As shown in FIGS. 1 and 2, the front face 3 of the object 1 has adesirable line along which the object should be cut 5 for cutting theobject 1. The line along which the object should be cut 5 is a linearlyextending virtual line (the object 1 may also be formed with an actualline acting as the line along which the object should be cut 5). In thelaser processing in accordance with this embodiment, the object 1 isirradiated with laser light L such that a light-converging point P ispositioned within the object 1 under a condition causing multiphotonabsorption, so as to form a modified region 7. Here, thelight-converging point is a location where the laser light L isconverged.

The laser light L is relatively moved along the line along which theobject should be cut 5 (in the direction of arrow A), so as to move thelight-converging point P along the line along which the object should becut 5. This forms the modified region 7 along the line along which theobject should be cut 5 only within the object 1 as shown in FIGS. 3 to5, and the modified region 7 forms a starting point region for cutting(part to cut)₈. In the laser processing method in accordance with thisembodiment, no modified region 7 is formed upon heating the object 1 bycausing the object 1 to absorb the laser light L. Instead, the laserlight L is transmitted through the object 1, so as to generatemultiphoton absorption within the object 1, thereby forming the modifiedregion 7. Hence, the laser light L is hardly absorbed by the front face3 of the object 1, whereby the front face 3 of the object 1 does notmelt.

If a start point exists at a location to cut when cutting the object 1,the object 1 fractures from this start point and thus can be cut with arelatively small force as shown in FIG. 6. This makes it possible to cutthe object 1 without generating unnecessary fractures in the front face3 of the object 1.

There seem to be the following two ways of cutting the object from thestarting point region for cutting acting as a start point. The firstcase is where, after forming the starting point region for cutting, anartificial force is applied to the object, so in that the objectfractures from the starting point region for cutting acting as a startpoint, whereby the object is cut. This is the cutting in the case wherethe object has a large thickness, for example. The application of anartificial force encompasses application of bending stress and shearingstress along the starting point region for cutting of the object, andexertion of a temperature difference upon the object to generate thermalstress, for example. The other case is where a starting point region forcutting is formed, so that the object is naturally fractured in across-sectional direction (thickness direction) of the object from thestarting point region for cutting acting as a start point, whereby theobject is cut. This is enabled, for example, by forming the startingpoint region for cutting by a single row of modified regions when theobject has a small thickness, and by a plurality of rows of modifiedregions aligned in the thickness direction when the object has a largethickness. Even in the case of natural fracturing, fractures do notextend to the front face at a location not formed with the startingpoint region for cutting in the part to cut, whereby only the partcorresponding to the location formed with the starting point region forcutting can be fractured. Thus, fracturing can be regulated well. Such afracturing method with favorable controllability is quite effective,since objects to be processed such as silicon wafers have recently beenapt to become thinner.

The modified region formed by multiphoton absorption in this embodimentincludes the following cases (1) to (3):

(1) Case where the Modified Region is a Crack Region Including One or aPlurality of Cracks

An object to be processed (e.g., glass or a piezoelectric material madeof LiTaO₃) is irradiated with laser light while a light-converging pointis positioned therewithin under a condition with an electric fieldintensity of at least 1×10⁸ (W/cm²) at the light-converging point and apulse width of 1 μs or less. This pulse width is a condition under whicha crack region can be formed only within the object while generatingmultiphoton absorption without causing unnecessary damages to theobject. This generates a phenomenon of optical damage due to multiphotonabsorption within the object. This optical damage induces thermaldistortion within the object, thereby forming a crack regiontherewithin. The upper limit of electric field intensity is 1×10¹²(W/cm²), for example. The pulse width is preferably 1 ns to 200 ns, forexample. The forming of a crack region due to multiphoton absorption isdescribed, for example, in “Internal Marking of Glass Substrate bySolid-state Laser Harmonics”, Proceedings of 45th Laser MaterialsProcessing Conference (December 1998), pp. 23-28.

The inventors determined relationships between the electric fieldintensity and the magnitude of crack by an experiment. Conditions forthe experiment are as follows:

(A) Object to be processed: Pyrex (registered trademark) glass (having athickness of 700 μm)

(B) Laser

-   -   Light source: semiconductor laser pumping Nd:YAG laser    -   Wavelength: 1064 nm    -   Laser light spot cross-sectional area: 3.14×10⁻⁸ cm²    -   Oscillation mode: Q-switch pulse    -   Repetition frequency: 100 kHz    -   Pulse width: 30 ns    -   Output: output <1 mJ/pulse    -   Laser light quality: TEM₀₀    -   Polarization characteristic: linear polarization

(C) Light-converging lens

-   -   Transmittance with respect to laser light wavelength: 60%

(D) Moving speed of a mounting table mounting the object: 100 mm/sec

Here, the laser light quality being TEM₀₀ indicates that the lightconvergence is so high that light can be converged up to about thewavelength of laser light.

FIG. 7 is a graph showing the results of the above-mentioned experiment.The abscissa indicates peak power density. Since laser light is pulselaser light, its electric field intensity is represented by the peakpower density. The ordinate indicates the size of a crack part (crackspot) formed within the object processed by one pulse of laser light.Crack spots gather, so as to form a crack region. The size of a crackspot refers to that of the part of dimensions of the crack spot yieldingthe maximum length. The data indicated by black circles in the graphrefers to a case where the light-converging lens (C) has a magnificationof ×100 and a numerical aperture (NA) of 0.80. On the other hand, thedata indicated by white circles in the graph refers to a case where thelight-converging lens (C) has a magnification of ×50 and a numericalaperture (NA) of 0.55. It is seen that crack spots begin to occur withinthe object when the peak power density reaches about 10¹¹ (W/cm²), andbecome greater as the peak power density increases.

A mechanism by which the object to be processed is cut upon formation ofa crack region in the laser processing in accordance with thisembodiment will now be explained with reference to FIGS. 8 to 11. Asshown in FIG. 8, the object 1 is irradiated with laser light L whilepositioning the light-converging point P within the object 1 under acondition where multiphoton absorption occurs, so as to form a crackregion 9 therewithin along a line along which the object should be cut.The crack region 9 is a region including one or a plurality of crackspots. The crack region 9 forms a starting point region for cutting. Asshown in FIG. 9, the crack further grows while using the crack region 9as a start point (i.e., using the starting point region for cutting as astart point). As shown in FIG. 10, the crack reaches the front face 3and rear face 21 of the object 1. As shown in FIG. 11, the object 1breaks, so as to be cut. The crack reaching the front face and rear faceof the object may grow naturally or grow as a force is applied to theobject.

(2) Case where the Modified Region is a Molten Processed Region

An object to be processed (e.g., a semiconductor material such assilicon) is irradiated with laser light while a light-converging pointis positioned therewithin under a condition with an electric fieldintensity of at least 1×10⁸ (W/cm²) at the light-converging point and apulse width of 1 μs or less. As a consequence, the inside of the objectis locally heated by multiphoton absorption. This heating forms a moltenprocessed region within the object. The molten processed region refersto a region once melted and then re-solidified, a region just in amelted state, or a region in the process of re-solidifying from itsmelted state, and may also be defined as a phase-changed region or aregion having changed its crystal structure. The molten processed regionmay also be regarded as a region in which a certain structure haschanged into another structure in monocrystal, amorphous, andpolycrystal structures. Namely, it refers to a region in which amonocrystal structure has changed into an amorphous structure, a regionin which a monocrystal structure has changed into a polycrystalstructure, and a region in which a monocrystal structure has changedinto a structure including an amorphous structure and a polycrystalstructure, for example. When the object is a silicon monocrystalstructure, the molten processed region is an amorphous siliconstructure, for example. The upper limit of electric field intensity is1×10¹² (W/cm²), for example. The pulse width is preferably 1 ns to 200ns, for example.

By an experiment, the inventors have verified that a molten processedregion is formed within a silicon wafer. Conditions for the experimentare as follows:

(A) Object to be processed: silicon wafer (having a thickness of 350 μmand an outer diameter of 4 inches)

(B) Laser

-   -   Light source: semiconductor laser pumping Nd:YAG laser    -   Wavelength: 1064 nm    -   Laser light spot cross-sectional area: 3.14×10⁻⁸ cm²    -   Oscillation mode: Q-switch pulse    -   Repetition frequency: 100 kHz    -   Pulse width: 30 ns    -   Output: 20 μJ/pulse    -   Laser light quality: TEM₀₀    -   Polarization characteristic: linear polarization

(C) Light-converging lens

-   -   Magnification: ×50    -   N. A.: 0.55    -   Transmittance with respect to laser light wavelength: 60%

(D) Moving speed of a mounting table mounting the object: 100 mm/sec

FIG. 12 is a view showing a photograph of a cut section in a part of asilicon wafer cut by laser processing under the above-mentionedconditions. A molten processed region 13 is formed within a siliconwafer 11. The size of the molten processed region 13 formed under theabove-mentioned conditions is about 100 μm in the thickness direction.

The fact that the molten processed region 13 is formed by multiphotonabsorption will now be explained. FIG. 13 is a graph showingrelationships between the wavelength of laser light and thetransmittance within the silicon substrate. Here, respective reflectingcomponents on the front face side and rear face side of the siliconsubstrate are eliminated, whereby only the transmittance therewithin isrepresented. The above-mentioned relationships are shown in the caseswhere the thickness t of the silicon substrate is 50 μm, 100 μm, 200 μm,500 μm, and 1000 μm, respectively.

For example, it is seen that laser light is transmitted through thesilicon substrate by at least 80% at 1064 nm, where the wavelength ofNd: YAG laser is located, when the silicon substrate has a thickness of500 μm or less. Since the silicon wafer 11 shown in FIG. 12 has athickness of 350 μm, the molten processed region 13 due to multiphotonabsorption is formed near the center of the silicon wafer, i.e., at apart separated from the front face by 175 μm. The transmittance in thiscase is 90% or greater with reference to a silicon wafer having athickness of 200 μm, whereby the laser light is absorbed within thesilicon wafer 11 only slightly and is substantially transmittedtherethrough. This means that the molten processed region 13 is notformed by laser light absorption within the silicon wafer 11 (i.e., notformed upon usual heating with laser light), but by multiphotonabsorption. The forming of a molten processed region by multiphotonabsorption is described, for example, in “Processing CharacteristicEvaluation of Silicon by Picosecond Pulse Laser”, Preprints of theNational Meeting of Japan Welding Society, No. 66 (April 2000), pp.72-73.

Here, a fracture is generated in the cross-sectional direction whileusing a molten processed region as a start point, whereby the siliconwafer is cut when the fracture reaches the front face and rear face ofthe silicon wafer. The fracture reaching the front face and rear face ofthe silicon wafer may grow naturally or grow as a force is applied tothe silicon wafer. The fracture naturally grows from the starting pointregion for cutting to the front face and rear face of the silicon waferin any of the cases where the fracture grows from the molten processedregion in a melted state and where the fracture grows from the moltenprocessed region in the process of re-solidifying from the melted state.In any of these cases, the molten processed region is formed only withinthe silicon wafer. In the cut section after cutting, the moltenprocessed region is formed only therewithin as shown in FIG. 12. When amolten processed region is formed within the object, unnecessaryfractures deviating from a line along which the object should be cut arehard to occur at the time of fracturing, which makes it easier tocontrol the fracturing.

(3) Case where the Modified Region is a Refractive Index Change Region

An object to be processed (e.g., glass) is irradiated with laser lightwhile a light-converging point is positioned therewithin under acondition with an electric field intensity of at least 1×10⁸ (W/cm²) atthe light-converging point and a pulse width of 1 ns or less. Whenmultiphoton absorption is generated within the object with a very shortpulse width, the energy caused by multiphoton absorption is nottransformed into thermal energy, so that a permanent structural changesuch as ionic valence change, crystallization, or polarizationorientation is induced within the object, whereby a refractive indexchange region is formed. The upper limit of electric field intensity is1×10¹² (W/cm²), for example. The pulse width is preferably 1 ns or less,more preferably 1 ps or less, for example. The forming of a refractiveindex change region by multiphoton absorption is described, for example,in “Formation of Photoinduced Structure Within Glass by FemtosecondLaser Irradiation”, Proceedings of 42th Laser Materials ProcessingConference (November 1997), pp. 105-111.

The cases of (1) to (3) are explained as modified regions formed bymultiphoton absorption in the foregoing. When a starting point regionfor cutting is formed as follows in view of the crystal structure of awafer-like object to be processed, the cleavage property thereof, andthe like, the substrate can be cut with a smaller force and a higheraccuracy while using the starting point region for cutting as a startpoint.

Namely, in the case of a substrate made of a monocrystal semiconductorhaving a diamond structure such as silicon, the starting point regionfor cutting is preferably formed in a direction along the (111) plane(first cleavage plane) or (110) plane (second cleavage plane). In thecase of a substrate made of a III-V family compound semiconductor havinga zinc ore type structure such as GaAs, the starting point region forcutting is preferably formed in a direction along the (110) plane. Inthe case of a substrate having a hexagonal crystal structure such assapphire (Al₂O₃), a starting point region for cutting is preferablyformed in a direction along the (1120) plane (A plane) or (1100) plane(M plane) while using the (0001) plane (C plane) as a principal plane.

When the substrate is formed with an orientation flat along a directionto be formed with the starting point region for cutting (e.g., in adirection along the (111) plane in the monocrystal silicon substrate) ora direction orthogonal to the direction to be formed with the startingpoint region for cutting, the starting point region for cuttingextending along the direction to be formed with the starting pointregion for cutting can be formed in the substrate in an easy andaccurate manner with reference to the orientation flat.

A laser processing apparatus used in the above-mentioned laserprocessing method will now be explained with reference to

FIG. 14. FIG. 14 is a schematic diagram of the laser processingapparatus 100.

The laser processing apparatus 100 comprises a laser light source 101for generating laser light L; a laser light source controller 102 forcontrolling the laser light source 101 so as to regulate the output,pulse width, etc. of laser light L and the like; a dichroic mirror 103,arranged so as to change the orientation of the optical axis of laserlight L by 90°, having a function of reflecting the laser light L; alight-converging lens 105 for converging the laser light L reflected bythe dichroic mirror 103;

a mounting table 107 for mounting an object to be processed 1 irradiatedwith the laser light L converged by the light-converging lens 105; anX-axis stage 109 for moving the mounting table 107 in the X-axisdirection; a Y-axis stage 111 for moving the mounting table 107 in theY-axis direction orthogonal to the X-axis direction; a Z-axis stage 113for moving the mounting table 107 in the Z-axis direction orthogonal tothe X- and Y-axis directions; and a stage controller 115 for controllingthe movement of these three stages 109, 111, 113.

This movement of light-converging point P in X(Y)-axis direction iseffected by moving the object 1 in the X(Y)-axis direction by theX(Y)-axis stage 109 (111). The Z-axis direction is a directionorthogonal to the front face 3 of the object 1, and thus becomes thedirection of focal depth of laser light L incident on the object 1.Therefore, moving the Z-axis stage 113 in the Z-axis direction canposition the light-converging point P of laser light L within the object1. This can place the light-converging point P at a desirable positionsuch as the substrate, the laminate part on the substrate, or the likein the object 1 when the object 1 has a multilayer structure, forexample.

The laser light source 101 is an Nd:YAG laser generating pulse laserlight. Known as other kinds of laser usable as the laser light source101 include Nd:YVO₄ laser, Nd:YLF laser, and titanium sapphire laser.Though pulse laser light is used for processing the object 1 in thisembodiment, continuous wave laser light may be used as long as it cancause multiphoton absorption.

The laser processing apparatus 100 further comprises an observationlight source 117 for generating a visible light beam for irradiating theobject 1 mounted on the mounting table 107, and a visible light beamsplitter 119 disposed on the same optical axis as that of the dichroicmirror 103 and light-converging lens 105. The dichroic mirror 103 isdisposed between the beam splitter 119 and light-converging lens 105.The beam splitter 119 has a function of reflecting about a half of avisual light beam and transmitting the remaining half therethrough, andis arranged so as to change the orientation of the optical axis of thevisual light beam by 90°. About a half of the visible light beamgenerated from the observation light source 117 is reflected by the beamsplitter 119, and thus reflected visible light beam is transmittedthrough the dichroic mirror 103 and light-converging lens 105, so as toilluminate the front face 3 of the object 1 including the line alongwhich the object should be cut 5 and the like. When the object 1 ismounted on the mounting table 107 such that the rear face of the object1 faces the light-converging lens 105, the “front face” mentioned abovebecomes the “rear face” as a matter of course.

The laser processing apparatus 100 further comprises an image pickupdevice 121 and an imaging lens 123 which are disposed on the sameoptical axis as that of the beam splitter 119, dichroic mirror 103, andlight-converging lens 105. An example of the image pickup device 121 isa CCD camera. The reflected light of the visual light beam havingilluminated the front face 3 including the line along which the objectshould be cut 5 and the like is transmitted through the light-converginglens 105, dichroic mirror 103, and beam splitter 119 and forms an imageby way of the imaging lens 123, whereas thus formed image is captured bythe image pickup device 121, so as to yield imaging data.

The laser processing apparatus 100 further comprises an imaging dataprocessor 125 for inputting the imaging data outputted from the imagepickup device 121, an overall controller 127 for controlling the laserprocessing apparatus 100 as a whole, and a monitor 129. According to theimaging data, the imaging data processor 125 calculates focal point datafor positioning the focal point of the visible light generated from theobservation light source 117 onto the front face 3 of the object 1.According to the focal point data, the stage controller 115 controls themovement of the Z-axis stage 113, so that the focal point of visiblelight is positioned on the front face 3 of the object. Hence, theimaging data processor 125 functions as an autofocus unit. Also,according to the imaging data, the imaging data processor 125 calculatesimage data such as an enlarged image of the front face 3. The image datais sent to the overall controller 127, subjected to various kinds ofprocessing therein, and then sent to the monitor 129. As a consequence,an enlarged image or the like is displayed on the monitor 129.

Data from the stage controller 115, image data from the imaging dataprocessor 125, and the like are fed into the overall controller 127.According to these data as well, the overall controller 127 regulatesthe laser light source controller 102, observation light source 117, andstage controller 115, thereby controlling the laser processing apparatus100 as a whole. Thus, the overall controller 127 functions as a computerunit.

The starting point region for cutting forming step in the case using theabove-mentioned laser processing apparatus will now be explained withreference to FIGS. 14 and 15. FIG. 15 is a flowchart for explaining thestarting point region for cutting forming step.

Light absorption characteristics of the substrate of the object 1 aredetermined by a spectrophotometer or the like which is not depicted.According to the results of measurement, a laser light source 101generating laser light L having a wavelength to which the substrate ofthe object 1 is transparent or exhibits a low absorption is chosen(S101). Subsequently, in view of the thickness and refractive index ofthe object 1, the amount of movement of the object 1 in the Z-axisdirection in the laser processing apparatus 100 is determined (S103).This is an amount of movement of the object 1 in the Z-axis directionwith reference to the light-converging point P of laser light Lpositioned at the rear face of the object 1 in order for thelight-converging point P of laser light L to be placed at a desirableposition within the object 1. This amount of movement is fed into theoverall controller 127.

The object 1 is mounted on the mounting table 107 of the laserprocessing apparatus 100 such that the rear face of the substrate facesthe light-converging lens 105. Subsequently, the thickness of the object1 is measured. According to the result of measurement of thickness andthe refractive index of the object 1, the amount of movement of theobject 1 in the Z-axis direction is determined (S103). This is an amountof movement of the object 1 in the Z-axis direction with reference tothe light-converging point of laser light L positioned at the front face3 of the object 1 in order for the light-converging point P of laserlight L to be positioned within the object 1. This amount of movement isfed into the overall controller 127.

The objet 1 is mounted on the mounting table 107 of the laser processingapparatus 100. Subsequently, visible light is generated from theobservation light source 117, so as to illuminate the front face of theobject 1 (S105). The illuminated front face 3 of the object 1 includingthe line along which the object should be cut 5 is captured by the imagepickup device 121. The imaging data captured by the imaging device 121is sent to the imaging data processor 125. According to the imagingdata, the imaging data processor 125 calculates such focal point datathat the focal point of visible light from the observation light source117 is positioned at the front face 3 (S 107).

The focal point data is sent to the stage controller 115. According tothe focal point data, the stage controller 115 moves the Z-axis stage113 in the Z-axis direction (S109). As a consequence, the focal point ofvisible light from the observation light source 117 is positioned at thefront face 3 of the object 1. According to the imaging data, the imagingdata processor 125 calculates enlarged image data of the front face 3 ofthe object 1 including the line along which the object should be cut 5.The enlarged image data is sent to the monitor 129 by way of the overallcontroller 127, whereby an enlarged image of the line along which theobject should be cut 5 and its vicinity is displayed on the monitor 129.

Movement amount data determined in step S103 has been fed into theoverall controller 127 beforehand, and is sent to the stage controller115. According to the movement amount data, the stage controller 115causes the Z-axis stage 113 to move the object 1 in the Z-axis directionto a position where the light-converging point P of laser light L ispositioned within the object 1 (S111).

Subsequently, laser light L is generated from the laser light source101, so as to irradiate the line along which the object should be cut 5in the front face 3 of the substrate of the object 1. Since thelight-converging point P of the laser light L is positioned within theobject 1, a modified region is formed only within the object 1. Then,the X-axis stage 109 and Y-axis stage 111 are moved along the line alongwhich the object should be cut 5, such that the modified region formedalong the line along which the object should be cut 5 forms a startingpoint region for cutting within the object 1 along the line along whichthe object should be cut 5 (S113).

The present invention will now be explained more specifically withreference to Examples.

Example 1

Example 1 of the method of cutting an object to be processed inaccordance with the present invention will be explained. FIGS. 17, 18,and 22 to 24 are partial sectional views of the object to be processed 1taken along the line XVII-XVII of FIG. 16. FIGS. 19 to 21 are partialsectional views of the object 1 taken along the line XIX-XIX of FIG. 16.

As shown in FIGS. 16 and 17, the front face 3 of the object to beprocessed 1, which is a silicon wafer, is formed with a plurality offunctional devices 17 in a matrix in parallel with an orientation flat16 of the object 1, whereby the object 1 is produced. Formed on thefront face 3 side of the object 1 is an insulating film 18 made of SiO₂or the like, which covers the front face 3 and functional devices 17.

Therefore, the object 1 is a substrate, whereas the functional devices17 and insulating film 18 constitute a laminate part disposed on thefront face of the substrate. Here, the laminate part disposed on thefront face of the substrate refers to one deposited on the front face ofthe substrate, one bonded onto the front face of the substrate, oneattached to the front face of the substrate, etc., regardless of whetherits material is different from or identical to that of the substrate.The laminate part disposed on the front face of the substrate includesone disposed in close contact with the substrate, one disposed with agap from the substrate, etc. Examples of the laminate part includesemiconductor active layers formed by crystal growth on the substrate,functional devices (which refer to light-receiving devices such asphotodiodes and light-emitting devices such as laser diodes, circuitdevices formed as a circuit, etc.) formed on the substrate, glasssubstrates bonded onto other glass substrates, etc. The laminate partalso includes one in which a plurality of layers are formed frommaterials different from each other.

Subsequently, as shown in FIG. 18, an expandable expansion film 19 isattached to the rear face 21 of the object 1, and then the object 1 ismounted on the mounting table 107 of the above-mentioned laserprocessing apparatus 100, for example, such that the front face 3 sideof the object 1 faces the light-converging lens 105. Thereafter, theobject 1 is irradiated with laser light L while its light-convergingpoint P is positioned within the object 1, so as to form a modifiedregion 7 within the object 1, and cause the modified region 7 to form astarting point region for cutting 8 along a line along which the objectshould be cut 5 inside by a predetermined distance from the front face 3(laser light incident face) of the object 1 (starting point region forcutting forming step). Since the object to be processed 1 is a siliconwafer, a molten processed region is formed as the modified region 7.

In the starting point region for cutting forming step, as shown in FIG.19, the starting point region for cutting 8 deviated from a center lineL passing the center position of the object 1 in the thickness directiontoward the front face (one end face) 3 is formed along the line alongwhich the object should be cut 5. In the case where the object 1, whichis a silicon wafer, has a thickness of 100 μm, by way of example, thewidth in the thickness direction (hereinafter simply be referred to as“width”) of an unmodified region 1 a positioned on the front face 3 sideof the starting point region for cutting 8 is 20 μm, the width of thestarting point region for cutting 8 (i.e., modified region 7) is 40 μm,and the width of an unmodified region 1 b positioned on the rear face 21side of the starting point region for cutting 8 is 40 μm. When thethickness of the object is 50 μm, the width of the unmodified region 1 ais 10 μm, the width of the starting point region for cutting 8 is 20 μm,and the width of the unmodified region 1 b is 20 μm.

In addition to such a “case where the starting point region for cutting8 is positioned across the center line CL”, a mode of “the startingpoint region for cutting 8 deviated from the center line CL toward thefront face 3” include the following two cases, for example. Namely,there are “a case where the whole starting point region for cutting 8 ispositioned on the front face 3 side of the center line CL” as shown inFIG. 20, and “a case where two starting point regions for cutting 8 a, 8b are formed on the front face 3 side and the rear face 21 side, suchthat the starting point region for cutting 8 b on the rear face 21 sideis positioned on the center line CL, whereas the starting point regionfor cutting 8 a on the front face 3 side is positioned between thestarting point region for cutting 8 b and the front face 3”.

In the case of FIG. 20, for example, the thickness of the object 1 is100 μm, the width of the unmodified region 1 a is 30 μm, the width ofthe starting point region for cutting 8 is 10 μm, and the width of theunmodified region 1 b is 60 μm. In the case of FIG. 21, the thickness ofthe object 1 is 200 μm, the width of the unmodified region 1 a is 20 μm,the width of the starting point region for cutting 8 a is 40 μm, thewidth of the unmodified region 1 c positioned between the starting pointregions for cutting 8 a, 8 b is 20 μm, the width of the starting pointregion for cutting 8 b is 40 μm, and the width of the unmodified region1 b is 80 μm.

In the starting point region for cutting forming step, the line alongwhich the object should be cut 5 is scanned with the laser light L. Theline along which the object should be cut 5 is set like a grid passingbetween functional devices 17, 17 adjacent each other (see FIG. 16). Thepositional data of the line along which the object should be cut 5 withrespect to the object 1 is stored into a storage section in the overallcontroller 127 in the laser processing apparatus 100, for example.

After the starting point region for cutting is formed, as shown in FIG.22, a knife edge 23 as pressing means is pressed against the object 1from the rear face (other end face) 21 side thereof by way of theexpansion film 19, so as to generate a fracture 24 from the startingpoint region for cutting 8 acting as a start point, and cause thefracture 24 to reach the front face 3 and rear face 21 of the objet 1(pressing step). As a consequence, the object 1 is divided intoindividual semiconductor chips 25 each having one functional device 17.

In the pressing step, the positional data of the line along which theobject should be cut 5 stored in the storage section is read out, andthe knife edge 23 is controlled according to the positional data, so asto be pressed against the object 1 along the line along which the objectshould be cut 5, whereby the object 1 is pressed along the line alongwhich the object should be cut 5.

Thus, the positional data of the line along which the object should becut with respect to the object 1 is stored in the starting point regionfor cutting forming step, and the object 1 is pressed against the linealong which the object should be cut 5 according to the positional datain the pressing step, whereby the pressing force can act easily andaccurately on the starting point region for cutting 8 formed within thesubstrate 1. Then, pressing the object 1 along the line along which theobject should be cut 5 can accurately cut the object 1 into eachfunctional device 17 while substantially eliminating the action of thepressing force on the functional devices 17.

When the modified region 7 is positioned near the front face 3 of theobject 1 as in the pressing step shown in FIG. 22, the knife edge 23 ispressed against the rear face 21 of the object 1 along the startingpoint region for cutting (part to cut) 8 formed by the modified region7, so as to break and cut the object 1. This is because a large tensilestress among bending stresses generated upon pressing the knife edge 23acts on the modified region 7, whereby the object 1 can be cut with arelatively small force.

After the object 1 is pressed, the expansion film 19 is expandedoutward, so as to separate the semiconductor chips 25 from each other asshown in FIG. 23. Separating the semiconductor chips 25 from each otherby using the expansion film 19 as such can make it easier to pick up thesemiconductor chips 25.

In the method of cutting an object to be processed in accordance withExample 1, as explained in the foregoing, the modified region 7 formedby multiphoton absorption forms the region to cut 8 within the object 1along the line along which the object should be cut 5. Here, themultiphoton absorption occurs locally within the object 1, so that thelaser light L is hardly absorbed by the front face 3 and rear face 21 ofthe object 1, whereby the front face 3 and rear face 21 can be preventedfrom melting upon irradiation with the laser light L. Since the regionto cut 8 is formed so as to shift from the center line CL of the object1 toward the front face 3, when the object 1 is pressed by the knifeedge 23 from the rear face 21 side, the fracture can be generated in theobject 1 from the starting point region for cutting 8 acting as a startpoint by a smaller pressing force than in the case where the startingpoint region for cutting 8 is formed on the center line CL. This canaccurately cut the object 1 along the line along which the object shouldbe cut 5 while preventing unnecessary fractures deviating from the linealong which the object should be cut 5 from occurring.

In the case where a metal film for electrostatic prevention or the likeis formed between adjacent functional devices 17, 17 (i.e., on the linealong which the object should be cut 5) in the object 1 shown in FIGS.16 and 17, so that the object 1 is hard to be irradiated with the laserlight L from the front face 3 side, the starting point region forcutting 8 can be formed as follows. Namely, as shown in FIG. 24, aprotection film 20 for protecting the functional devices 17 is attachedto the front face 3 side of the object 1 before attaching the expansionfilm 19, and the object 1 is mounted on the mounting table 107 of theabove-mentioned laser processing apparatus 100, for example, such thatthe rear face 21 side of the object 1 faces the light-converging lens105. Then, the object 1 is irradiated with the laser light L while thelight-converging point P is positioned therewithin, so as to form amodified region 7 within the object 1, and cause the modified region 7to form a starting point region for cutting 8 shifted from the centerline CL to the front face 3 side of the object 1 along the line alongwhich the object should be cut 5.

Example 2

Example 2 of the method of cutting an object to be processed inaccordance with the present invention will now be explained. FIGS. 25 to27 are partial sectional views of the object 1 taken along the lineXVII-XVII of FIG. 16.

As in Example 1 mentioned above, the object to be processed 1 shown inFIGS. 16 and 17 is produced, and a starting point region for cutting 8is formed along a line along which the object should be cut 5 inside bya predetermined distance from the front face 3 (laser light incidentface) of the object 1 (starting point region for cutting forming step).In the starting point region for cutting forming step in Example 2, thestarting point region for cutting 8 shifted from the center line CLpassing the center position of the object 1 in the thickness directiontoward the rear face (one end face) 21 is formed along the line alongwhich the object should be cut 5 as shown in FIG. 25.

Subsequently, as shown in FIG. 26, a protective film 20 is attached tothe front face 3 side of the object to be processed 1, so as to coverthe functional devices 17. Then, the knife edge 23 is pressed againstthe object 1 from the front face (other face) 3 side of the object 1 byway of the expansion film 19, so as to generate a fracture 24 from thestarting point region for cutting 8 acting as a start point, and causethe fracture 24 to reach the front face 3 and rear face 21 of the object1 (pressing step). As a consequence, the object 1 is divided intoindividual semiconductor chips 25 each having one functional device 17.

In the pressing step, as in Example 1, the positional data of the linealong which the object should be cut 5 stored in the storage section isread out, and the knife edge 23 is controlled according to thepositional data, so as to be pressed against the object 1 along the linealong which the object should be cut 5, whereby the object 1 is pressedalong the line along which the object should be cut 5.

When the modified region 7 is positioned near the rear face 21 of theobject 1 as in the pressing step shown in FIG. 26, the knife edge 23 ispressed against the front face 3 of the objet 1 along the starting pointregion for cutting (part to cut) 8 formed by the modified region 7, soas to break and cut the object 1. This is because a large tensile stressamong bending stresses generated upon pressing the knife edge 23 acts onthe modified region 7, whereby the object 1 can be cut with a relativelysmall force.

Subsequently, the protective film 20 is peeled off from the object 1,and the expansion film 19 is expanded outward, so as to separate thesemiconductor chips 25 from each other as in Example 1, whereby thesemiconductor chips 25 are picked up.

In the method of cutting an object to be processed in accordance withExample 2, as explained in the foregoing, the starting point region forcutting 8 is formed so as to shift from the center line CL of the object1 toward the rear face 21. Therefore, when the knife edge 23 presses theobject 1 from the front face 3 side, the fracture 24 can be generated inthe object 1 from the starting point region for cutting 8 acting as astart point by a smaller pressing force than in the case where thestarting point region for cutting 8 is formed on the center line CL.This can accurately cut the object 1 along the line along which theobject should be cut 5 while preventing unnecessary fractures deviatingfrom the line along which the object should be cut 5 from occurring.Also, since the object 1 can be cut with a small pressing force, theinfluence on the functional devices 17 when the objet 1 is pressed fromthe front face 3 side can be alleviated.

In the case where a metal film for electrostatic prevention or the likeis formed between adjacent functional devices 17, 17 in the object 1, sothat the object 1 is hard to be irradiated with the laser light L fromthe front face 3 side, the object 1 is irradiated with the laser light Lwhile the light-converging point P is positioned therewithin, so as toform a modified region 7 within the object 1, and cause the modifiedregion 7 to form a starting point region for cutting 8 shifted from thecenter line CL toward the rear face 21 of the object 1 along the linealong which the object should be cut 5 by a method similar to that ofExample 1 mentioned above as shown in FIG. 27.

The present invention is not limited to the above-mentioned embodiment.For example, though the front face 3 side or rear face 21 side of theobject 1 is pressed along the line along which the object should be cut5 in the pressing step of Examples 1 and 2, the object 1 as a whole onthe front face 3 side or rear face 21 side may be pressed with a rolleror the like. Since the fracture 24 is generated from the starting pointregion for cutting 8 acting as a start point, the object 1 canefficiently be cut along the line along which the object should be cut 5in this case as well. Also, parts (e.g., respective parts of thefunctional devices 17) of the object 1 on the front face 3 side or rearface 21 side may successively be pressed with a pressure needle or thelike. Means for pressing the object 1 along the line along which theobject should be cut 5 includes not only the above-mentioned knife edge23, but also a cutter.

INDUSTRIAL APPLICABILITY

As explained in the foregoing, the method of cutting an object to beprocessed in accordance with the present invention can accurately cutthe object to be processed.

1-3. (canceled)
 4. A method of cutting an object having a wafer-likeshape, the object having a laminated portion including a functionaldevice on a front face of the object, and a protective film attached tothe front face of the object covering the functional device, the methodcomprising: a modified region forming step of irradiating the objectwith laser light while positioning a focusing point of the laser lightwithin the object, thereby forming modified region within the objectalong a cutting line along which the object is to be cut, the modifiedregion being deviated from a center position of the object in athickness direction of the object towards a rear face of the object; anda pressing step of pressing the object from the front face of the objectthrough the protective film to cut the object into chips, after themodified region forming step.
 5. The method according to claim 4,wherein in the modified region forming step, the laser light enters bythe rear face of the object and the modified region is formed to bedeviated from the center position of the object in the thicknessdirection of the object towards the rear face of the object.
 6. Themethod according to claim 4, further comprising: an expansion filmattachment step of attaching an expansion film to the rear face of theobject before the pressing step, and after the modified region formingstep.
 7. The method according to claim 6, further comprising: a step ofexpanding the expansion film to separate the chips from each other, thechips previously formed by the cutting of the object by the pressingstep; and a step of peeling off the separated chips from the expansionfilm, after the pressing step and the expanding step.
 8. A method ofmanufacturing a semiconductor device from a wafer-like object, themethod comprising: a modified region forming step of irradiating thewafer-like object with laser light, while positioning a focusing pointof the laser light within the wafer-like object, thereby forming amodified region within the object along a cutting line along which thewafer-like object is to be cut, the modified region being deviated froma center position of the object in a thickness direction of thewafer-like object towards a rear face of the object, the wafer-likeobject having at least one semiconductor device arranged on a front faceof the wafer-like object, and a protective film covering the at leastone semiconductor device and the front face of the wafer-like object;and a pressing step of pressing the wafer-like object from the frontface of the wafer-like object through the protective film to cut theobject into semiconductor devices so that the wafer-like object is cutalong the cutting line to provide semiconductor devices, after themodified region forming step.
 9. The method according to claim 8,wherein in the modified region forming step, the laser light enters intothe rear face of the wafer-like object and the modified region is formedto be deviated from the center position of the object in the thicknessdirection of the wafer-like object towards the rear face of thewafer-like object.
 10. The method according to claim 8, furthercomprising: an expansion film attachment step of attaching an expansionfilm to the rear face of the wafer-like object, before the pressing stepand after the modified region forming step.
 11. The method according toclaim 10, further comprising: a step of expanding the expansion film toseparate the semiconductor devices from each other, the semiconductordevices being previously formed by the cutting of the wafer-like objectby the pressing step; and a step of peeling off the separatedsemiconductor devices from the expansion film, after the pressing stepand the expanding step.